What Are the Barriers Inhibiting North South Travel?

There is a series of natural barriers extending across europe and asia which inhibit north south travel. The most obvious of these barriers lies in the west, and it's the mediterranean sea. As we move eastward, the next natural barrier lies just to the east of the mediterranean, and that's the black sea. Then as we look east from the black sea, we have another sea, the caspian sea. And as we moveEastward from the caspan sea we find the vast deserts and arid regions of central asia. Even though some limited travel through this reg was possible, the rugged and dry terrain made it very difficult for any extended period of travel